Some of the malware memory resident programs will lock out registry edits as well. Spybot does that by default, unsure if MSE can be set that way. Possible that a program isn’t giving you an info pop-up when it blocks the write? Agree with Andrea that it’s probably something not allowing a registry write.

(I would wonder though, is there a possibility that Tabbles may be not able to ‘see’ the reg entry for some reason?)